After the discovery of carbon nanotubes in 1991, scientific efforts have been devoted to the production of carbon nanotubes in higher yields; the production of carbon nanotubes with consistent dimensions, e.g., diameter and length; processes which separate nanotubes from other reaction products; processes which eliminate the entanglement of tubes with each other and the development of useful applications.
Currently, carbon nano particles including both nanotubes and monofilaments are found in extended commercial applications in modern technologies, for example, for manufacture of composite materials, nanoscale machines, flat panel displays, and computer memory devices. The wide application of carbon nanotubes is based on their unique physical and mechanical properties, which show the high electrical and thermal conductivity, and high strength along the nanotubes' axis.
Their high aspect ratio, mechanical resilience and excellent electrical conduction make them ideal for probe microscopy tips. There are several different types of scanning probe microscopy, including scanning tunneling microscopy (STM), scanning force microscopy (SFM), atomic force microscopy (AFM), magnetic force microscopy (MFM), and magnetic resonance force microscopy (MRFM). Nanotubes have previously been made into atomic force microscopy (AFM) tips and have proven to have great advantages in imaging and manipulation over conventional silicon and silicon nitride tips. AFM instruments are well known for producing images with resolution in the nanometer or smaller range. AFM resolution is dependent on physical characteristics of the scanning probe including composition, size, shape and rigidity of the probe. Both length and width (or diameter) of the probe affect the resolution because, for example, the length limits the maximum depth of a detail that may be measured, and the width limits the minimum breadth of a detail that may be measured. Silicon probes are commonly used, but have a tip diameter generally greater than 10 nm, and are easily damaged or worn during use. Scanning probes made of carbon nanotubes have been shown to be acceptable alternatives to silicon probes and are known to be mechanically stable.
U.S. Pat. No. 6,582,673 issued Jun. 24, 2003 contributed a more consistent, predictable method for manufacturing a particular configuration of carbon nanotubes as well as providing a solution to the problems associated with handling and manipulating the “small” wand which is only visible with high-power electron microscopes, or other costly visual aids. Through the process disclosed therein, a “graphitic outer layer” defined as a carbon material comprising one or more distinct structures, is intentionally formed during the carbon nanotubes production and becomes an integral part of the carbon nanotubes device.
However, there are no easy and controllable methods to attach a carbon nanotube to a scanning probe tip, due to the extremely small size of the carbon nanotubes. Previous approaches have included the mechanical attachment of a CNT onto an AFM tip, chemical vapor deposition growth of a CNT directly onto commercial atomic force microscope made of Si or one of its derivatives, and electric or magnetic field induced multi-wall nanotube probe attachment.
U.S. patent application Ser. No. 10/961,929 filed on Oct. 8, 2004 discloses a more consistent and controlled method for attaching a novel carbon nanotube probe to the SPM cantilever tip using Focus Ion Beam (FIB) technology. Through the method of the present invention, a FIB tool is used to form a slot in the SPM cantilever tip and the carbon nanotube probe is inserted into the formed slot. The inserted carbon nanotube probe is welded to the SPM cantilever tip using the FIB tool to deposit metal atoms to the joint between the carbon nanotube probe and the cantilever tip, thus welding the carbon nanotube probe to the cantilever tip.
Since the pioneer work on the field emission from carbon nanotubes there have been intense studies of the field emission properties of the carbon nanotubes. Among the work on field emission from individual nanotubes, most of the work was carried out inside a transmission (or scanning) electron microscope on a substrate that contains plural carbon nanotubes. A microprobe is then used to select one particular carbon nanotube for the field emission study. While this method is useful for field emission studies, it is not practicable for real applications. The few field emission measurements carried out using a fabricated individual nanotubes emitter typically employed an optical or electron microscope to attach a carbon nanotubes or a carbon nanotubes bundle to an nickel (Ni) or tungsten (W) tip with conducting glue or use of van de Waals force. Through these studies, it has been established that the carbon nanotubes is an ideal electron field emitter with the following advantages over the prior art: (1) high brightness, (2) low energy spread, (3) emission current stability, and (4) long lifetime.
The following approaches have been used for the fabrication of individual carbon nanotubes field emitters. Mechanical attachment using van der Waals forces was developed in 1995. A DC arc method was used to produce a nanotubes containing boule which was then baked in air to etch away all but the best nanotube material. Then individual nanotubes were attached to an 8 μm diameter graphitic fiber electrode through van der Waals force. The graphitic fiber was then attached to a stainless steel electrode with silver paint. A SEM micrograph reveals that the nanotube is not an individual nanotube, but a stalk of 5 to 10 multi-wall nanotubes adhered together. Using this technique, it was extremely difficult to fabricate a nanotube electron emitter due to its nanometer size.
Direct growth of multiple wall carbon nanotubes on a Ni catalyst was developed in 2002. Following this technique, high resolution electron beam lithography was used to pattern the Ni catalyst on a substrate. The spacing between the catalyst was between 10 and 100 μm. Using PECVD with a C2H2 and a NH3 gas mixture, 5 μm long multi-wall nanotubes having a width of 60 nm were grown on a substrate. A scanning anode field emission microscope with a probe ball diameter of 100 μm was used to measure the emission current from individual carbon nanotube tips. Although it was claimed that the emission current was from an individual nanotube, the geometry is similar to that of a carbon nanotube film having an array of nanotubes.
Using a mechanical attachment technique, conductive carbon tape is applied to the tip of an electrochemically etched tungsten wire with a 200 nm radius tip and, under an optical microscope, individual nanotubes are mounted on the tungsten tip with the carbon tape.
Another prior art technique used a scanning electron microscope to pick up and attach carbon nanotubes to a support by irradiation. During pick up, the e-beam dissociates residual organic species. The deposits are strong but it is hard to evaluate the electrical properties of the contacts.
The present invention contributes a more consistent and controlled method for using Focus Ion Beam (FIB) technology to fabricate an electron field emitter. The FIB technique is a standard, well-developed semiconductor industry technique. It is very controllable, and using the technique, our carbon nanotube with a graphitic outer layer (see U.S. Pat. No. 6,582,673 B1) can be attached to a tip of a tungsten wire approximately along the axis of the tungsten wire. Through the method of the present invention, a 0.20 mm diameter tungsten wire is etched to a sharp tip and a FIB tool is used to fabricate a slot on the sharp tip of the tungsten wire. A carbon fiber with a nanotube tip is wielded to the tungsten tip.